1. Field of the Invention
The invention relates to a device for processing the modulated signals received by a sonar system and more particularly a side-looking sonar system;
2. Description of the Prior Art
Side-looking sonar systems generally comprise one or more transmitting and receiving transducers fixed to the hull of a ship or a profiled body called a fish, which is towed in an immersed state by the ship. Each of these transducers comprises a plurality of transducers aligned in a direction parallel to the longitudinal axis of the ship or fish, so that its directional pattern has a very small opening angle on each side of a vertical transverse plane and a wider opening angle in the same plane. The transmitting transducers are energized at regular intervals for sending sound pulses in an oblique direction to the bottom of the water. These pulses irradiate or "insonifies" the bottom on one side or on both sides of the ship or fish along a narrow band elongated perpendicularly to the ship route and are backscattered to the surface.
The receiving transducer or the receiving transducers, because of their directional pattern, are adapted for picking up preferably the echoes coming from the irradiated bands.
The signals received by the transducers, in response to the transmitted pulses, are amplified preferably with an amplification gain increasing with time, so as to correct the amplitude variations due to the obliqueness of the propagation paths, and then are detected so as to obtain a low frequency signal representative of the envelope of these signals.
Then the demodulated signals are recorded as a function of the propagation time so as to obtain transverse sweeping of the irradiated zone. The demodulated signals are used for example for varying as a function of time the intensity of the spot of an optical recorder or of a cathode ray tube. The transmission-reception cycles are repeated regularly as the ship advances and the successive records are juxtaposed so as to obtain a relatively continuous image of the bottom. A lateral sonar is described for example in French Pat. No. 2 064 4400.
Detection or recovery of the envelope of the signals received is very often obtained in sonar systems by rectifying said signals and the information which they contain relative to the phase variations is in general not used.
The signals received are affected, with respect to the transmitted signals, by rapidly varying phase shifts characteristic of the modifications of the diffraction or back scattering power of the irradiated surface and slow varying phase shifts due to the Doppler effect and resulting from the relative speed, with respect to the moving ship, of the portions of this surface situated outside the transverse plane of symmetry of the transmitter beams. The non-use of the phase data means that no distinction can be made on the image between the unevennesses of the bottom situated in the plane of symmetry of the beam and outside this plane.
The processing device of the invention is adapted to take into account the phase variations of the back scattered signals and consequently allows a finer representation of the bottom surface.